I picked Mord. Take into consideration the fact that the Darkin have to be strong of will first, before they can ever manifest their strength in their host. Willpower is exactly what Mordekaiser is based off of, he became a revenant after merely "refusing to fade". He just said "no", and walked right back into life TWICE. He's a hulking armor giant with no flesh, so a massive bsword could bash him around a bit, but Mord has no real life essence to feed off of, so most of Aatrox "anti-life" power is nullified by Morde's literal anti-life. I normally would retaliate to that with Aatrox being physically dead too, but his host, however, is not, and is made of twisted and reformed flesh and carapace.
Onto the confrontation itself, I can see Mordy being taken off guard a bit by such a powerful opponent, but he wouldn't falter, knowing he'd simply return with a better plan if he failed. So he would stand, and he would fight, and he would laugh and Aatrox clangs away at his armor, and does some damage, but only to dent metal. Mord would, presumably, either smash the hell out of the host and leave the sword forgotten once more, or he would take the sword and either: 1) Wield it and dominate Aatrox's mind similarly to Kayn/Varus(world ending scenario), or 2) Keep it in his dead kingdom of Mitna Rachnun. Regardless, Morde would, in my opinion, succeed because his very existence is cheating.
